1. Field of the Invention
The present invention relates generally to a rubber composition of an inner liner of a tire, and more particularly to improvements in an inner liner for a tire whose internal pressure is required to be securely maintained as particularly in radial-ply tires and bias-ply tires for passenger cars, tires for motorcycles, and tires for trucks and buses.
2. Description of the Prior Art
In connection with radial-ply and bias-ply tires for passenger cars, tires for motorcycles, and tires for trucks and buses, it is important to maintain the internal pressure of the tires. For this purpose, a rubber composition including as a main component a halogenated butylene rubber has been hitherto used as an inner liner of the tire as disclosed, for example, in Japanese Patent Provisional (First) Publication Nos. 57-172945 and 57-195760.
Such a rubber composition including as the main component halogenated butylene rubber is useful from a view point of maintaining the internal pressure of the tire. However, the rubber composition is inferior in stickiness due to the fact that its main component is halogenated butylene rubber, so that sufficiently tight contact cannot be obtained between the inner liner and a tire casing during fabrication of a green tire. This will unavoidably allow air to enter between the inner liner and the tire casing after vulcanization. Additionally, sufficient adhesive strength cannot be obtained between the inner liner and the tire casing, thereby causing peeling of the inner liner from the tire casing. Furthermore, because of insufficiently tight contact of the inner liner with the tire casing, lubricant and the like applied before vulcanization for the purpose of making tight contact with a vulcanization bladder unavoidably enters between the inner liner and the tire casing, so that sufficient adhesive strength cannot be obtained between the inner liner layer and the tire casing. This will allow cracks to be formed or extended at a location where the lubricant and the like has entered, thereby causing breakdown of the inner liner and the tire casing.
In this regard, it is usual to blend a suitable amount of a tackifier in order to increase the stickiness of the rubber composition of the inner liner. Examples of the tackifier are phenol resins, terpene resins, petroleum hydrocarbon resins and the like. However, these resins unavoidably harden at low temperatures and therefore adversely affect the low temperature characteristics of the inner liner, thereby forming cracks in the inner liner during vehicle cruising at low temperatures. Additionally, there is a tendency for the breaking strength of the rubber of the inner liner to decrease as the blending amount of these resins increases, thus forming cracks in the inner liner during vehicle cruising.
Furthermore, a resin such as brown asphalt has been hitherto blended in order to increase the adhesive strength of the inner liner with the tire casing in a tire fabrication process. However, such a resin readily displaces to the surface of the inner liner and crystallizes out there. This also lowers the stickiness of the inner liner.